TypeHoles demo.
Note that this program runs, and prints a type error when a hole _ is encountered at runtime.
Code copied from haskell.org/haskellwiki/GHC/TypeHoles
To run in IDE, after opening in IDE, go to Settings (hamburger icon in top-right) and set
- Environment: GHC 7.8
- Compiler flags: -fdefer-type-errors
{-# LANGUAGE TypeHoles #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
module FreeMonad where
-- Please go to Settings (hamburger icon in top-right) and set
-- Environment: GHC 7.8
-- Compiler flags: -fdefer-type-errors
data Free f a
= Pure a
| Free (f (Free f a))
instance Functor f => Monad (Free f) where
return a = Pure a
Pure a >>= f = f a
Free f >>= g = Free _ -- Hole!
instance Show (f (Free f a)) => Show (Free f a) where
show (Free x) = show x
demo :: Free [] String
demo = Free [] >>= return
main :: IO ()
main = print demo -- Fails with type error at runtime, since `demo` tries to call `_`
